arXiv subjects

Ilari J. Maasilta

Publications and source records attributed to Ilari J. Maasilta.

18 recordsLinked to original sources

arXiv2026

Coherent control of thermal transport with pillar-based phononic crystals

Two-dimensional phononic crystals (PnCs) formed by a periodic array of holes in a suspended membrane have previously been used to coherently control thermal conductance at low temperatures by modifying the phonon dispersion, thereby altering the phonon group velocities and the density of states. Here, in contrast, we demonstrate that PnCs formed by a periodic array of Al pillars on an uncut \SiN membrane can also be used to achieve similar coherent control. We have measured and simulated the thermal conductance of four pillar-based PnCs with different lattice constants ranging from 0.3 to 5 $μ$m at sub-Kelvin temperatures, showing a strong up to an order of magnitude reduction in thermal conductance compared to an unaltered membrane. For the larger lattice constants $> 1 $ $μ$m, however, the experiments do not agree with the coherent theory simulations, which we interpret as a breakdown of coherence induced by increasingly effective diffusive scattering due to the roughness of the Al pillar surfaces.

cond-mat.mes-hall↗

arXiv2026

Phonon-blocked junction calorimeter

This study introduces the theory of a microcalorimeter based on phonon-blocked superconducting tunnel junctions, integrating on-chip electron cooling and boundary resistance phonon isolation to achieve exceptional energy resolution and rapid thermal response. A general theoretical framework is presented, along with derived approximate analytical expressions for key performance metrics, including cooling factor, thermal time constant, noise equivalent power and energy resolution. The work examines the influence of device parameters, including non-ideal effects such as subgap tunneling and heat backflow, and offers insights into optimizing the detector performance. The findings highlight the potential of the phonon-blocked junction microcalorimeters to rival or even outperform state-of-the-art technologies such as transition-edge sensors, paving the way for applications requiring precise and fast energy spectroscopy.

physics.ins-det↗

arXiv2024

Highly tunable NbTiN Josephson junctions fabricated with focused helium ion beam

We demonstrate a direct writing method for the fabrication of planar Josephson junctions from high quality superconducting niobium titanium nitride (NbTiN) thin films, by creating local disorder using focused He-ion beam irradiation in a helium ion microscope. We show that we can control the suppression of superconductivity in NbTiN as a function of the helium ion beam fluence, enabling us to successfully fabricate Josephson junctions with highly tunable weak links ranging from metallic to insulating phase, due to the continuous nature of the disorder-induced superconductor-insulator transition. We demonstrate the successful fabrication of both SNS and SIS type of devices, and show that we can achieve exceptionally wide range ($\sim$5 orders of magnitude) of critical current densities and junction resistances. The SNS type junctions follow closely the ideal resistively and capacitively shunted junction behavior, have high characteristic voltages up to $\sim$ 1.5\,mV and show Shapiro steps up to very high orders, while the SIS junctions also follow established theories well. The results suggest that junctions fabricated with this method from NbTiN are suitable for a wide range of applications in superconducting electronics because of the excellent mechanical, electrical and microwave properties of NbTiN. In particular, as NbTiN has previously been used to fabricate high quality factor microwave resonators, we see the method as a promising and simple way to realize superconducting qubits and other quantum devices using only a single superconducting film.

cond-mat.supr-con↗

arXiv2023

Superconductor-ferromagnet hybrids for non-reciprocal electronics and detectors

We review the use of hybrid thin films composed of superconductors and ferromagnets for creating non-reciprocal electronic components and self-biased detectors of electromagnetic radiation. We begin by introducing the theory behind these effects, as well as discussing various potential materials that can be used in the fabrication of these components. We then proceed with a detailed discussion on the fabrication and characterization of Al/EuS/Cu and EuS/Al/Co-based detectors, along with their noise analysis. Additionally, we suggest some approaches for multiplexing such self-biased detectors.

cond-mat.supr-con↗

arXiv2023

Complete tunneling of acoustic waves between closely spaced piezoelectric crystals

When two piezoelectric solids are placed in close proximity, acoustic waves (phonons) can "tunnel" across a vacuum gap transmitting energy between the two solids. Here, we demonstrate analytically that not only is such a phenomenon possible, but that a simple resonance condition exists for which complete transmission of the incoming wave is possible, physically corresponding to the excitation of leaky surface waves. This result is derived for an arbitrary anisotropic crystal symmetry and orientation. We also show that the complete transmission condition can be related to the surface electric impedance and the effective surface permittivity of the piezoelectric material, making it possible to be determined experimentally. In addition, we present numerical results for the maximum power transmittance of a slow transverse wave, tunneling between identical ZnO crystals, as function of all possible crystal orientations. The results show a large range of orientations for which complete tunneling can be achieved.

cond-mat.mes-hall↗

arXiv2023

Heat transfer across a vacuum gap induced by piezoelectrically mediated acoustic phonon tunneling

In contradictin to the common concept that acoustic phonons can only travel inside a material medium, they can in fact "tunnel" across a vacuum gap with the help of piezoelectricity, transmitting a significantly stronger heat flux than that of blackbody radiation. Here, we present a theoretical formulation for the heat flux of such piezoelectrically mediated heat transfer, applicable to any anisotropic piezoelectric crystals with an arbitrary orientation. A few numerical results are demonstrated and compared to heat transfer driven by other close-range mechanisms, including near-field radiative heat transfer and other acoustic phonon tunneling mechanisms. We find that piezoelectrically mediated heat transfer has a significant effect when the vacuum gap size is smaller than the phonon characteristic thermal wavelength, and its heat flux can dominate heat transfer between piezoelectric solids over all other known heat transfer mechanisms at temperatures below 50 K.

cond-mat.mes-hall↗

arXiv2022

Analytical models for the pulse shape of a superconductor-ferromagnet tunnel junction thermoelectric microcalorimeter

The superconductor-ferromagnet thermoelectric detector (SFTED) is a novel ultrasensitive radiation detector based on the giant thermoelectric effect in superconductorferromagnet tunnel junctions. We demonstrate analytical models and solutions in the time domain for a SFTED operated as a microcalorimeter (pulse excitation), in the linear smallsignal limit. Based on these solutions, the signal current and temperature pulse response were studied for two different electrical circuit models, providing design conditions for stable and non-oscillatory response.

cond-mat.supr-con↗

arXiv2022

Acoustic wave tunneling across a vacuum gap between two piezoelectric crystals with arbitrary symmetry and orientation

It is not widely appreciated that an acoustic wave can "jump" or "tunnel" across a vacuum gap between two piezoelectric solids, nor has the general case been formulated or studied in detail. Here, we remedy that situation, by presenting a general formalism and approach to study such an acoustic tunneling effect between two arbitrarily oriented anisotropic piezoelectric semi-infinite crystals. The approach allows one to solve for the reflection and transmission coefficients of all the partial wave modes, and is amenable to practical numerical or even analytical implementation, as we demonstrate by a few chosen examples. The formalism can be used in the future for quantitative studies of the tunneling effect in connection not only with the manipulation of acoustic waves, but with many other areas of physics of vibrations such as heat transport, for example.

cond-mat.mes-hall↗

arXiv2021

Controlling thermal conductance using three-dimensional phononic crystals

Controlling thermal transport at the nanoscale is vital for many applications. Previously, it has been shown that this control can be achieved with periodically nanostructured two-dimensional phononic crystals, for the case of suspended devices. Here we show that thermal conductance can also be controlled with three-dimensional phononic crystals, allowing the engineering of the thermal contact of more varied devices without the need of suspension in the future. We show experimental results measured at sub-Kelvin temperatures for two different period three-dimensional crystals, as well as for a bulk control structure. The results show that the conductance can be enhanced with the phononic crystal structures in our geometry. This result cannot be fully explained by the simplest theory taking into account the coherent modification of the phonon band structure, calculated with finite element method simulations.

cond-mat.mes-hall↗

arXiv2020

Superconducting tunnel junction fabrication on three-dimensional topography via direct laser writing

Superconducting junctions are widely used in multitude of applications ranging from quantum information science and sensing to solid-state cooling. Traditionally, such devices must be fabricated on flat substrates using standard lithographic techniques. In this study, we demonstrate a highly versatile method that allows for superconducting junctions to be fabricated on a more complex topography. It is based on maskless direct laser writing (DLW) two-photon lithography, which allows writing in 3D space. We show that high-quality normal metal-insulator-superconductor (NIS) tunnel junctions can be fabricated on top of a 20 $μ$m tall three-dimensional topography. Combined with more advanced resist coating methods, this technique could allow sub-micron device fabrication on almost any type of topography in the future.

cond-mat.supr-con↗

arXiv2020

Superconductor-ferromagnet tunnel junction thermoelectric bolometer and calorimeter with a SQUID readout

Superconductor-ferromagnet thermoelectric detector (SFTED) is a novel ultrasensitive radiation detector based on the giant thermoelectric effect in superconductor-ferromagnet tunnel junctions. This type of detector can be operated without the need of additional bias lines, and is predicted to provide a performance rivaling transition-edge sensors and kinetic inductance detectors. Here we report our numerical studies on the SFTED noise equivalent power, energy resolution and time constant, and the feasibility of a SQUID readout in both bolometric and calorimetric regimes, with the goal to provide practical design parameters for the detector fabrication and the readout circuitry implementation.

physics.ins-det↗

arXiv2019

A Composite Phononic Crystal Design for Quasiparticle Lifetime Enhancement in Kinetic Inductance Detectors

A nanoscale phononic crystal filter (reflector) is designed for a kinetic inductance detector where the reflection band is matched to the quasiparticle recombination phonons with the aim to increase quasiparticle lifetime in the superconducting resonator. The inductor is enclosed by a 1 um wide phononic crystal membrane section with two simple hole patterns that each contain a partial spectral gap for various high frequency phonon modes. The phononic crystal is narrow enough for low frequency thermal phonons to propagate unimpeded. With 3D phonon scattering simulations over a 40 dB attenuation in transmitted power is found for the crystal, which was previously estimated to give a lifetime enhancement of nearly two orders of magnitude.

physics.ins-det↗

arXiv2019

Minimizing coherent thermal conductance by controlling the periodicity of two-dimensional phononic crystals

Periodic hole array phononic crystals (PnC) can strongly modify the phonon dispersion relations, and have been shown to influence thermal conductance coherently, especially at low temperatures where scattering is suppressed. One very important parameter influencing this effect is the period of the structure. Here, we measured the sub-Kelvin thermal conductance of nanofabricated PnCs with identical hole filling factors, but three different periodicities, 4, 8, and 16 $μ$m, using superconducting tunnel junction thermometry. We found that all the measured samples can suppress thermal conductance by an order of magnitude, and have a lower thermal conductance than the previously measured smaller period, 1 $μ$m and 2.4 $μ$m structures. The 8 $μ$m period PnC gives the lowest thermal conductance of all the samples above, and has the lowest specific conductance/unit heater length observed to date in PnCs. In contrast, coherent transport theory predicts that the longest period should have the lowest thermal conductance. Comparison to incoherent simulations suggests that diffusive boundary scattering is likely the mechanism behind the partial breakdown of the coherent theory.

cond-mat.mes-hall↗

arXiv2019

Domain wall induced modulation of low field H-T phase diagram in patterned superconductor-ferromagnet stripes

We present a systematic study of the magnetic domain wall induced modulation of superconducting transition temperature (Tc) in Nb/Ni bilayer stripes. By varying the thickness of the Ni layer from 20 nm to 100 nm we have been able to measure the low field Tc-H phase diagram spanning the Neel domain wall and Bloch domain wall range of thicknesses. Micromagnetic simulations and magnetic force microscopy measurements confirmed a stronger out-of-plane stray field in the Bloch domain walls compared to the Neel walls. A suppression in Tc was observed in the magnetization reversal region of the Ni film, the magnitude of which followed linearly to the strength of the out-of-plane stray field due to the domain walls. The magnitude of the stray field was quantified by comparing the Tc of the suppressed region of H-Tc phase diagrams with the unaffected part of the H-Tc curve. With Bloch domain walls a change in Tc of more than 60 mK was observed which is much more compared to the earlier reports. We believe that the narrow stripe geometry of the bilayers and the transverse external field maximized the effect of the domain walls in the Ni layer on the overlying superconducting film, leading to a larger change in Tc. This observation may be useful for domain wall controlled switching devices in superconducting spintronics.

cond-mat.supr-con↗

arXiv2016

InAs nanowire superconducting tunnel junctions: spectroscopy, thermometry and nanorefrigeration

We demonstrate an original method -- based on controlled oxidation -- to create high-quality tunnel junctions between superconducting Al reservoirs and InAs semiconductor nanowires. We show clean tunnel characteristics with a current suppression by over $4$ orders of magnitude for a junction bias well below the Al gap $Δ_0 \approx 200\,μ{\rm eV}$. The experimental data are in close agreement with the BCS theoretical expectations of a superconducting tunnel junction. The studied devices combine small-scale tunnel contacts working as thermometers as well as larger electrodes that provide a proof-of-principle active {\em cooling} of the electron distribution in the nanowire. A peak refrigeration of about $δT = 10\,{\rm mK}$ is achieved at a bath temperature $T_{bath}\approx250-350\,{\rm mK}$ in our prototype devices. This method opens important perspectives for the investigation of thermoelectric effects in semiconductor nanostructures and for nanoscale refrigeration.

cond-mat.mes-hall↗

arXiv2013

Statistical Characterization of Self-Assembled Colloidal Crystals by Single-Step Vertical Deposition

We have statistically characterized the self-assembly of multi-layer polystyrene colloidal crystals, using the technique of vertical deposition, with parameters chosen to produce thick layers of self-assembled crystals in one deposition step. In addition, using a lithographically directed self-assembly method, we have shown that the size of multi-layer, continuous crack-free domains in lithographically defined areas can be many times larger than in the surrounding areas. In a single deposition step, we have produced continuous colloidal crystal films of 260 nm diameter polystyrene spheres approximately 30 -- 40 layers thick, with a controllable lateral size of 80 -- 100 $\upmu$m without lithography, and as high as 250 $\upmu$m with the lithographic template.

cond-mat.mes-hall↗

arXiv2012

Complex impedance, responsivity and noise of transition-edge sensors: analytical solutions for two- and three-block thermal models

The responsivity and noise of a voltage-biased superconducting transition-edge sensor depends strongly on the details of its thermal model, and the simplest theory for TES response assumes a single heat capacity connected to the heat bath. Here, analytical results are derived and discussed for the complex impedance, the responsivity and the noise of a transition-edge sensor, when the thermal model is not simple but consists of either two or three connected heat capacities. The implications of the differences of the models are discussed, as well.

cond-mat.supr-con↗

arXiv2011

Effect of a thin AlO_x layer on transition-edge sensor properties

We have studied the physics of transition-edge sensor (TES) devices with an insulating AlOx layer on top of the device to allow implementation of more complex detector geometries. By comparing devices with and without the insulating film, we have observed significant additional noise apparently caused by the insulator layer. In addition, AlOx was found to be a relatively good thermal conductor. This adds an unforeseen internal thermal feature to the system.

cond-mat.supr-con↗